Ferroelectric materials can form the basis for data storage devices, where digital “1” and “0” levels are represented by the electric polarization of a ferroelectric film pointing “up” or “down”. Storage devices based on a ferroelectric storage medium include Ferroelectric Random Access Memory (FeRAM) and scanning probe storage systems (“FE-Probe”).
In an FeRAM memory cell the essential storage element includes a thin ferroelectric film sandwiched between fixed, conductive electrodes. To write a bit to such a cell, a voltage pulse of either positive or negative polarity is applied between the electrodes in order to switch the internal polarization of the ferroelectric film to the “up” or “down” state, respectively. To read back the data from the FeRAM cell, a read voltage of a certain polarity (e.g., positive) is applied, which switches the polarization of the ferroelectric film in cells storing a “0” (“down” polarization), while having no effect in cells storing a “1”. A sense amplifier measures the charge flow that results when the polarization switches, so that a current pulse is observed for cells which stored a “0”, but not for cells which stored a “1”, thus providing a destructive readback capability.
In an FE-Probe device, one of the electrodes (referred to as a “tip”) is movable relative to the media. In both cases the binary “1” and “0” are stored by causing the polarization of the ferroelectric film to point “up” or “down”, either in the entire cell in the case of FeRAM, or in a spatially small region (domain) local to the tip in the case of the FE-Probe. Data can then be read out destructively by applying a voltage of a magnitude and polarity such as to cause the polarization to point “up”. Cells (FeRAM) or domains (FE-Probe) polarized “down” (e.g., representing “0”), will then switch to the “up” state, and a charge will flow which is proportional to the remanent polarization of the ferroelectric film. Cells or domains polarized “up” will have no such current flow. The presence or absence of this current flow, as determined by a sense amplifier, can then be used to determine whether the cell or domain had contained a “1” or “0”. However, for a typical domain size of 25 nm×25 nm, desirable for an FE-Probe device, the resulting charge would be limited to about 6000 electrons, giving a current of about 1 nA for a read time of 1 microsecond, which makes high-speed, low error-rate readout difficult. In addition, the readback is necessarily destructive, i.e., not preserving the original data.
Probe storage devices have been proposed to provide small size, high capacity, low cost data storage devices. A scanning probe storage device based on ferroelectric media includes one or more heads, each including an electrode that moves relative to a ferroelectric thin film media. Binary “1's” and “0's” are stored in the media by causing the polarization of the ferroelectric film to point “up” or “down” in a spatially small region (domain) local to the electrode, by applying suitable voltages to the electrode. Data can then be read out by a variety of means, including sensing of piezoelectric surface displacement, measurement of local conductivity changes, or by sensing current flow during polarization reversal (destructive readout). Regardless of the readback mechanism, the head or heads should be mechanically robust, compatible with the ferroelectric media, provide intimate electrical proximity to the media, provide a ground plane to shield for noise, and include an area of hard insulator around the read/write electrode to allow the head to “fly” on lubricant and slow wear. Finally the heads need to be manufacturable by a process compatible with the integrated silicon-based electronic circuits required for readout in a practical storage device. Standard tips manufactured for Scanning Probe Microscopy (SPM) do not meet these requirements.
One of the challenges in designing probe storage devices is obtaining accurate position feedback for servo loops. Track densities in probe storage are much higher than in magnetic recording, with for example, up to 600 K tracks per inch (TPI). Such high track densities place great demands on the servo positioning system. In fact, for a 20 GB product it is estimated that positioning accuracy must be maintained to within 3 nm. Such accurate positioning requires extremely accurate sensing.
One of the challenges in the probe storage area is maintaining accurate spacing between the head and media wafers. The proposed spacing is 30 μm with head and media wafers that are 13×13 mm2. Variations in this spacing could modify the contact force, angle, and position of the probe head against the media wafer, thus potentially introducing noise in read and write and compromising the reliability of the head and the media mechanical interfacing.
Manufacturing tolerances are expected to result in static variations in head and media spacing from device to device. The stack-up tolerances may include head and media wafer thickness variation, adhesive thickness variation, and injection molding precision of the actuator and package. These tolerances could be as large as 10 or more microns.
Vibration and shock are expected to result in dynamic changes in the head and media spacing for a given device. The translation stage to which the media (or the head) substrate is attached is suspended by flexible springs, which allow large linear translation motions. Unfortunately these springs may also allow vertical motions and tilting motions in the presence of external disturbances. Depending on the stiffness of the support springs and the direction of the external forces, the head levers may bend a fraction of a micron to ten's of microns. In the worst case, the probe heads may lose contact with the media.
To date solutions have focused on using high-aspect-ratio springs (width-to-thickness>5) to passively maintain head-media spacing or actuators to actively control the media wafer. The high-aspect-ratio springs are difficult to manufacture using conventional (non-MEMS) technology and could not easily provide the required vertical stiffness and horizontal flexibility simultaneously. Active control (of vertical translation and two axes of tilt) requires additional mechanics and electronics for the actuators and control circuitry, which are prohibitive given the tight space and power budget.
There is a need for a probe storage apparatus that can achieve the required position sensing accuracy.